1. Field of the Invention
The present invention relates to a system and method for making electromagnetic measurements of a test object (also known as equipment under test (EUT)) using a transverse electromagnetic (TEM) cell. More specifically, the present invention relates to a system and method for maintaining the EUT in a fixed tilt position and, as contemplated by various embodiments, a horizontal (i.e., "gravity-down") position as the TEM cell is tilted and EUT rotated to specified angles. The electromagnetic readings taken at those angles are then processed to measure the electric field and the magnetic field produced by the EUT.
2. Related Information
Electrical devices such as computers, telephones, etc. all emit a certain amount of electromagnetic radiation, particularly in the radio frequency (RF) portion of the electromagnetic spectrum. More specifically, these devices produce an electric field as well as a magnetic field. Such fields can interfere with the operation of other electrical equipment in the vicinity and, at high enough intensities, can also cause harm to people.
Because of the unwanted effects of electromagnetic fields, it is important to test electrical devices prior to allowing them to be sold or otherwise used. In fact, many countries require that electrical devices meet certain standards before they can be sold. In the U.S., the Federal Communications Commission sets specific limits concerning the maximum amount of electromagnetic radiation that can be emitted by such devices. These limits can be found in 47 CFR (FCC Rules), Parts 15 and 18.
Over the years, various schemes and mechanisms have been devised to measure the electromagnetic radiation emanating from electrical devices. One such device developed in recent years uses a large "cell" into which the equipment under test (hereinafter EUT) is placed. The cell acts as a large coaxial cable, with the center wire running through the cell (in the form of a broad flat plate called a "septum") and the outer electrical casing encompassing the EUT. Electromagnetic measurements taken at one end of the cell give an indication of the electromagnetic radiation emitted from the EUT. The emissions from the EUT develop voltages between the cell septum and the outer cell walls. These voltages may be read to determine the magnitude and frequency of the emissions coming from the EUT.
The "cells" described above are often called Transverse Electromagnetic (TEM) cells. They are referred to as "transverse" because both the electric and magnetic field vectors are everywhere perpendicular to the wave normal. This reproduces the alignment of these fields at large distances from the EUT. TEM cells are described in greater detail in papers such as that written by M. L. Crawford: "Generation of Standard E M Fields Using TEM Transmission Cells," IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-16, No. 4, 1974 p.189-195, which is incorporated by reference herein. Modified TEM cells have been developed which can test for frequencies in the gigahertz range. Such cells include GTEM cells. Other types of cells include WTEM (wire TEM) cells.
The electromagnetic measurements taken from a TEM cell are a composite of the electric and magnetic fields along two specific vectors. Schemes must thus be devised to analyze this information and separate the electric field reading from the magnetic field reading. Often, electric and magnetic radiators are modeled as being comprised of three mutually orthogonal dipole moments (one corresponding to each of the x, y and z axes). The readings taken are analyzed to solve for these dipole moments of the radiating source.
In order to get an accurate reading of the electric and magnetic fields, conventional schemes have dictated that the EUT should be rotated and readings taken at various angles and along various axes. This is because TEM cells can only sense electric fields that are vertical and magnetic fields that are horizontal across the TEM cell's width. For example, the electric field of an antenna placed in a horizontal position will not be detected. Consequently, readings of the EUT must be taken at different positions.
An example of a scheme for analyzing the electromagnetic information taken at various angles and along various axes in a TEM cell has been set forth by Dr. Perry Wilson in "On Simulating OATS Near-Field Emission Measurements Via GTEM Cell Measurements" in the Record of the "1993 IEEE International Symposium on Electromagnetic Compatibility," Aug. 9-13, 1993, Dallas, Tex. pp 53-57, which is incorporated by reference herein. Dr. Wilson's scheme requires that electromagnetic measurements for 9 different positions of the EUT be taken. His scheme then analyzes the readings and yields three magnetic and three electric dipole moments (one corresponding to each of the x, y and z axes for the electric field and also for the magnetic field). In addition to higher frequencies, this scheme also works for electromagnetic emissions below 30 MHz, which are typically difficult to obtain.
A problem with Dr. Wilson's scheme is the very fact that it requires turning and tilting the EUT for taking certain readings. Such movements cause wires such as AC power cables (which act as antennas) that may be part of or attached to the EUT to orient themselves differently relative to the rest of the EUT. This movement is a problem, since the wires themselves can be a source of much electromagnetic radiation, and thus typically need to be taken into account when calculating an EUTs electric and magnetic field. If they become reoriented with respect to the EUT during testing, this reorientation will change the electromagnetic field being radiated, causing inaccuracies in the final results.
One partial solution developed by EMCO of Austin Tex., is to maintain a TEM cell at a tilt angle of 54.736 degrees. Since certain devices such as some printers and medical equipment will simply not work when tilted (which is another problem with schemes that tilt the EUT itself), a rotatable table is inserted into the TEM cell and positioned to maintain the EUT in a horizontal, gravity-down position. EMCO contemplates that the TEM cell can be maintained at this tilt angle and the horizontal table holding the EUT rotated to three different positions to determine the electric and magnetic fields. The scheme for determining the fields assumes a tilt angle of about 54.736 degrees. A discussion describing this can be found in an article by John D. M. Osburn and Edwin L. Bronaugh of EMCO, entitled "The `Hyper-Rotated` GHz Transverse Electromagnetic (GTEM) Cell, Expanding The Performance Envelope," and is incorporated by reference herein.
The problem with EMCO's approach is that, particularly where the impedance of the EUT is not already known, it has been determined that the readings obtained from this approach are not always as accurate as desired. This is, at least in part, because the number of viewpoints at which readings are taken is insufficient. In addition, this approach does not work for electromagnetic emissions below 30 MHz. Further, if readings at an angle other than 54.736 degrees were desired, the table used by EMCO would have to be manually readjusted for each angle, and a new scheme for determining the electric and magnetic fields would have to be implemented.
Thus, what is needed is a scheme for deriving sufficient information from a TEM cell to determine accurate results of the electric and magnetic fields of the EUT so as not to require tilting the EUT. The scheme should permit results to be obtained for "lower" frequencies (e.g., below the 30 MHz level). Further, such a scheme should reduce any reorientation of cables, such as AC power cables or data cables which must exit the cell, from causing errors in the ultimate field readings. If such a scheme necessitates tilting the TEM cell, what is then also needed is a device that maintains the EUT in a fixed tilt position, (e.g., a horizontal or "gravity-down" position) throughout the tilting, thus allowing the automation of the testing procedure.